Dual channel radars have been developed which provide simultaneous range and all-range operation. Examples of such radar systems are shown in Fishbein et al U.S. Pat. No. 3,562,750 entitled "Continuous Wave Correlation Radar", issued Feb. 9, 1971 and U.S. Pat. No. 3,568,188 to Fishbein et al entitled "Continuous Wave Radar with Means for Indicating Moving Target Direction", issued Mar. 2, 1971.
In such systems, doppler frequency signals from all moving targets within a single radar beam appear in the all-range channel and doppler frequency signals from targets at a selected (gated) range appear in the range gate channel. The all-range channel signals have more energy--because of the higher duty cycle--than the range gate channel signals. By correlating doppler signals from the two channels with a single correlator, information concerning target range and direction can be obtained. By correlating the signals in the all range and range gate channels, one achieves enhancement of the return from the range gate channel. The correlation in such systems involves the multiplication of the signal of poor range resolution with a signal of good range resolution. Such systems are not designed to penetrate clutter and foliage because only one frequency signal is transmitted and this one signal must be of relatively high frequency in order to obtain a narrow beam necessary for high angular resolution.
In the present invention, targets can be detected which are normally obscured by such disturbances as foliage, atmospheric precipitation and smog because of the relatively low frequency (say, L band) operation at which disturbance penetration capability is relatively high. The low frequency doppler signal at such a low frequency is readily detectable and is useful in providing target identification. Operation at a higher frequency, however, has the advantage of providing, with a relatively small antenna aperture, a narrow beam suitable for good angular resolution and allows for an improvement in signal-to-non-signal ratio. The dual frequency system of this invention is useful when the target and clutter is in the same resolution cell (a volume of space approximately equal to the cross section of the narrower beam.
The MTI doppler radar system shown in Goggins, Jr. U.S. Pat. No. 3,614,786, issued Oct. 19, 1971, uses a harmonic frequency phase signature technique for discriminating between moving and stationary targets. Unlike the present invention, Goggins, Jr. is not designed to distinguish between a target and clutter and noise. The dual frequency signals are applied to a simple phase detector and there is no long term integration in Goggins, Jr., much less two correlators. The Goggins, Jr. system requires a relatively good signal-to-noise ratio for both channels at all times to avoid spurious indications. The system of Goggins, Jr. unlike that of the invention, would not be satisfactory for target identification in those cases when the amplitude of the higher frequency target return is equal to or less than the amplitude of the higher frequency clutter return.
The two U.S. patents to Sletten et al, namely U.S. Pat. Nos. 3,719,945 and 3,719,946, both issued Mar. 6, 1973 and the U.S. Pat. No. 3,725,917, issued Apr. 3, 1973 to Sletten et al are not doppler radar systems, these systems involve two or more multiple coherent harmonic frequency signals which are compared in phase by a phase comparator to provide one or more sets of signals whose characteristics do not distinguish a target which is moving from other returns.
The previous comments made concerning the Goggins, Jr. patent also apply to the three Sletten et al patents mentioned above.